1. Field of the Invention
The present invention relates to an electric motor preferably used in a vehicle, and particularly, to improvement to balance cooling of the electric motor with lubrication thereof.
2. Description of Related Art
There has been known an electric motor including an shaft-core oil passage formed in a shaft core of a rotor shaft in a coaxial manner with the rotor shaft, and a radial oil passage provided in a penetrating manner toward a radial outer side of the rotor shaft from the shaft-core oil passage. In such an electric motor, a technique to realize effective cooling is suggested. An example of such a technique is a forcible oil-cooled electric motor described in Japanese Patent Application Publication No. 9-154258 (JP 9-154258 A). According to the technique, a tapered shaft-core oil passage is formed in a shaft core of the rotor shaft so that its oil input side has a small diameter and its oil output side has a large diameter, thereby making it possible to realize effective cooling.
In the meantime, oil to be supplied into the rotor shaft is used to cool down parts including a magnet in the electric motor, and is also .used to lubricate parts including bearings pivotally supporting the rotor shaft. However, in the conventional technique, even in a case where a necessity of cooling is relatively small, for example, even in a case where a rotation speed of the electric motor is relatively low, the oil is preferentially supplied to an oil passage for cooling, which may result in that the oil may not sufficiently spread over parts that require lubrication. In view of this, development of an electric motor that balances cooling of the electric motor with lubrication thereof has been demanded.